The SSY-1 is a small pulsed Nd:YAG laser originally used as part of the laser rangefinder in the M1 Abrams tank. The laser head and PFN-1 pulse forming network (flashlamp capacitor, inductor and diode) are readily available for under $250 as surplus on eBay and Meredith Instruments. For more information on the laser, a useful source of information is Sam's Laser FAQ.

With the laser head and pulse forming network in hand, all that is left to have a complete working laser is a power supply to charge the capacitor in the pulse forming network, and a high voltage trigger for the flashlamp. The power supply here fulfills these two roles. With the exception of a few resistors and the trigger transformer, I built this entirely from scrap parts I had, so none of the components are particularly special and may be substituted with a similar component. The case and main power transformer were originally part of a pulsed argon laser, and the power transformer just happened to be ideal for this power supply, and the case worked nicely as well. I originally wanted to have everything except the laser head in one box, but this case was far to small for the pulse forming network to fit inside, so now it sits externally with the laser head. The high voltage regulator transistor Q3 was originally the horizontal output transistor in a monitor, and I chose this transistor because it was literally the first one I picked out of my transistor bin with a high enough voltage rating. Any NPN transistor with a voltage rating at or above 1200V that can handle a few hundred milliamps should do.

Circuit Description

Click to enlarge:

The capacitor in the pulse forming network requires between 700-900VDC. This voltage is provided by rectifying the 700V winding of T1. Current limiting is provided by resistors R3-R5, and the voltage is regulated by Q3. The output voltage is divided by resistors R7 and R8 and compared to a zener diode D7 reference voltage that is adjusted by potentiometer R1. When the output voltage is low with respect to the reference, comparator IC2 turns on the regulator transistor Q3 via transistor Q2 and optoisolator OK1. Because the emitter of Q3 is at a high potential, an isolated 5V power supply is required to provide current to the base (note that in this schematic +5V is referenced to GND_2, where GND_2 is represented by downward pointing triangles). Potentiometer R1 may be adjusted to regulate the output voltage to any voltage up to over 900V (voltages above this may cause damage to the laser). When the external capacitors charge to the desired voltage, LED1 will light.

To trigger the flashlamp to fire, capacitor C8 is discharged through the trigger transformer T2 which generates a high voltage pulse. C8 is charged through the resistive divider formed by R18-R21, and activating relay SW3 allows the capacitor to discharge into the trigger transformer. The relay may be activated either by pressing a momentary toggle switch SW2, or externally triggered via TTL on BNC connection X1.